本邦の炭酸塩沈殿物を多量に伴う温泉・鉱泉の地化学的特徴 : CO_2地中貯留に対するナチュラルアナログの可能性

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Water qualities and occurrence of precipitates in 10 hot and mineral springs with large calcareous deposits in Japan were investigated in order to elucidate geochemical conditions of the precipitation of carbonate minerals and water-rock interactions in their reservoirs. Chemical analyses of water samples have revealed that the spring waters were rich in NaHCO3 and CO2 components. Out of 10 springs investigated, 2 springs contain Ca and CO2-related soluble components with genetic relations to the dissolution of limestone underground; 4 springs also enriched in Ca were green-tuff type affected by gypsum dissolution. Other springs poor in SO4 component have geochemical characteristics suggesting the contribution of deep-seated fluid. The pH of reservoir fluids is considered to be buffered by CO2-HCO3 speciation. This chemical condition probably promotes rock alteration including dissolution of calcite and plagioclase, resulting in formation of large calcareous deposits in the fields studied. The calcareous precipitates consist only of calcite in three fields with relatively low water temperatures; the other deposits contain aragonite with calcite possible due to their formation at high temperatures and high concentrations of Mg, Mn and SO4 in spring waters. From a comparison of water chemistry at the points of precipitation, logarithmic critical saturation indices (SI) are empirically derived to be 0.7-0.9 for calcium carbonate and over 3.5, 0.5, 1.2 and 3.4 for dolomite, magnesite, siderite and dawsonite, respectively. Distribution coefficients of Sr and Mg between spring water and precipitates indicate that calcareous precipitates were formed near equilibrium conditions. The possible ware-rock interaction processes elucidated in this study should be taken into account for geochemical modeling of CO2 geological reservoirs. The empirical values of SI, which are all positive for major carbonate minerals, can be the most important finding because geochemical simulation previously conducted commonly assumed precipitation at the point of saturation (SI=0).
2009-09-30

本邦の炭酸塩沈殿物を多量に伴う温泉・鉱泉の地化学的特徴 : CO_2地中貯留に対するナチュラルアナログの可能性

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